1. Field of the Invention
The invention herein relates to a catalyst for enhancing the conversion of the dehydrogenation reaction of aromatic hydrocarbons such as ethylbenzene, wherein carbon dioxide is used as an oxidant over a catalyst in which an active component of iron oxides is highly dispersed onto a zeolite, active carbon, .gamma.-alumina or silica carrier. Further, the invention relates to a dehydrogenation method of aromatic hydrocarbons by means of using said catalyst.
2. Description of the Prior Art
Styrene, one of aromatic hydrocarbons, manufactured by the dehydrogenation process of ethylbenzene is a very important compound and is widely used as a raw material and a monomer for synthetic rubber, ABS resin and polystyrene. Further, due to an increase in demand, the production amount of styrene has been grown year by year. Styrene is industrially manufactured by the dehydrogenation process of ethylbenzene via excess steam over an iron oxide-based catalyst. Alternatively, it can be manufactured by the epoxidation process of propylene and ethylbenzene hydroperoxide over a molybdenum-based catalyst. Among the aforementioned methods, the typical styrene manufacturing process involves the dehydrogenation process of ethylbenzene via the addition of steam and is responsible for 90% of the world styrene production.
Based on a rapid increase in demand of synthetic rubber, the research on the subject of the dehydrogenation reaction of ethylbenzene for mass production of styrene has been actively carried out from the early 1940's in the US and other parts of the world. In the case of the dehydrogenation process of ethylbenzene, which is the most widely used styrene manufacturing process, the process is operated by adding excess steam to ethylbenzene in an adiabatic reactor under pressurized condition with the reaction temperature of about 600.degree. C. In this process, the most widely known industrial catalyst is an iron oxide catalyst without a carrier, and the common constituents as core therein are Fe.sub.2 O.sub.3 and K.sub.2 O. As early as 1947, in U.S. Pat. No. 2,426,829 granted to Standard Oil Co. as assignee, the activity of an iron oxide catalyst in conjunction with an alkaline promoter during the dehydrogenation process of ethylbenzene via steam vapor was shown to be most active. Further, it is known that various catalyst promoters to added to the Fe--K based catalyst contribute to the enhancement of the catalytic activity, styrene selectivity and structural stability. For example, U.S. Pat. No. 5,023,225 teaches that cerium and chromium contribute to the enhancement of the catalytic activity and suggests that calcium, vanadium, molybdenum and tungsten contribute to the enhancement of selectivity while cerium and chromium contribute to the structural stability. Further, U.S. Pat. No. 5,190,906 granted to Nissan Girdler Catalyst Co. as assignee teaches that the activity with respect to the dehydrogenation of ethylbenzene using steam vapor increases when a small amount of titanium oxide is added to the K--Fe oxide catalyst.
On the other hand, as pointed out by Cavani and Trifiro, it is known that the problems seen in the other dehydrogenation process of paraffin are also observed in the dehydrogenation process of ethylbenzene (Appl. Catal., 133, 219 (1995)). The problems associated with the dehydrogenation process of ethylbenzene are as follows: thermodynamic limitation, low conversion rate, recycling of unreacted reactants, high endothermic energy (.DELTA.H.degree.=28.1 kcal/mol), and deactivation of a catalyst by coke formation. During the industrial dehydrogenation process of ethylbenzene, an excess steam is added for proper operation, and the necessary heat for reaction is partially provided by super-heated steam. Then, the partial pressure of ethylbenzene and hydrogen is reduced, which brings about a shift in the equilibrium towards the high conversion of ethylbenzene. Further, because the modification reaction of steam concurrently occurs at this point, the amount of coke or its precursor formed on the surface of a catalyst during the dehydrogenation reaction of ethylbenzene is reduced, which in turns significantly increases the lifetime of a catalyst. However, the process is still problematic due to the following factors: an increase in cost of energy due to the use of excess steam, consumption of ethylbenzene and styrene due to the side reaction of steam reforming, and a difficulty in controlling the oxidation state of the catalyst.
Various methods have been devised to overcome several problems associated with the use of steam during the dehydrogenation of ethylbenzene. The first method involves combining the dehydrogenation of ethylbenzene and the oxidation reaction of hydrogen. In this method, the dehydrogenated hydrogen is oxidized by oxygen in order to supply the heat of reaction and to modify the reaction equilibrium as deemed necessary. The second method involves lowering the reaction temperature by means of oxidative dehydrogenation via molecular oxygen, thereby converting the endothermic reaction to one of exothermic reaction. The third method is an attempt to lower the reaction temperature by improving the reaction equilibrium of the dehydrogenation of ethylbenzene, which is an equilibrium limitation reaction, by means of application of an inorganic membrane catalyst. Lastly, the fourth method is an attempt to increase the yield of styrene and the activity of dehydrogenation of ethylbenzene by using a mild oxidant, i.e., carbon dioxide.
As mentioned above, the first method can provide a shift in the reaction equilibrium to the direction of a high yield of styrene by means of continuously removing the produced hydrogen during the reaction. In actuality, when the reactants are passed through the three types of reactors using the above method, the conversion of ethylbenzene can reach 80% or above per cycle with the styrene selectivity not far off from that of the dehydrogenation process via steam (Appl., Catal., 133, 219 (1995)). In this process, two types of catalysts are mainly used. The first is the case in which a catalyst for dehydrogenation of ethylbenzene and a catalyst for hydrogen oxidation are used in two catalyst layers. The second type is the case in which one catalyst having two concurrent catalytic functions is used. U.S. Pat. No. 4,788,371 as assigned to UOP suggests the use of a catalyst containing Sn/K/Pt/Al.sub.2 O.sub.3 with the capacity of simultaneous dehydrogenation and hydrogen oxidation. Based on such method which enables the selective oxidation of hydrogen in production, it was further suggested that the SMART process be used with the enhanced process of dehydrogenation of ethylbenzene (Appl. Catal., 133, 219 (1995)).
In the second method of the oxidative dehydrogenation of ethylbenzene with molecular oxygen, there is an advantage in that not only the heat of reaction is exothermic (.DELTA.H.degree.=-29.7 kcal/mol), but also the reaction equilibrium can be markedly increased. However, since the oxidative dehydrogenation uses molecular, oxygen there is a danger of explosion due to the side reaction of the complete oxidation reaction and the violent reaction of oxygen and the reactants. The key to the second method is nevertheless the high selectivity of styrene of 90% or more. In order to increase the selectivity of oxidative dehydrogenation reaction, several procedures have been tried as follows: oxidative dehydrogenation over metal oxides having weak acidity or weak, oxidative dehydrogenation with a mild oxidant instead of oxygen, and oxidative dehydrogenation with an electrochemical method. Among these procedures, Vrieland was able to obtain a styrene selectivity close to 90% by applying various metal phosphates at the reaction temperature of 500.about.600.degree. C. (J. Catal., 111, 1 (1988)). Drago et al. disclosed that a high styrene selectivity of 90% in conjunction with a high conversion of ethylbenzene was obtained at a low reaction temperature of 350.degree. C. by using a carbon molecular sieve as catalyst (J. Mol. Catal., 58, 227 (1990); Appl. Catal., 112, 117 (1994)). Further, Hanuza et al. was able to obtain selectivity of 93.5% and conversion of 65% at 520.degree. C. when the reaction was carried out with the molar ratio of 1:1:8:20 with respect to benzene/oxygen/steam/nitrogen over a catalyst containing 9% V.sub.2 O.sub.5 (J. Mol. Catal., 29, 109 (1985)).
The third method, the application of a catalytic inorganic membrane reactor, can improve the conversion of ethylbenzene by favorably shifling the reaction equilibrium by means of introducing a hydrogen permselective membrane. In particular, UK Patent No. 2,201,159 suggests the use of a ceramic membrane reactor which can effectively separate hydrogen among the dehydrogenated products. Wu and Liu were able to increase the yield of styrene by combining K-promoted Fe.sub.2 O.sub.3 catalyst and inorganic membrane reactor (Ind. Eng. Chem. Res., 29, 232 (1990)). The method is superb in principle but has several disadvantages as follows: the expensive construction costs of facilities, the difficult commercialization of inorganic membrane reactor, and the inefficient heat and material transfer.
Meanwhile, in addition to these aforementioned methods, the dehydrogenation of ethylbenzene using carbon dioxide has been mentioned in the recent years. Carbon dioxide possesses a much weak oxidizability as compared to oxygen molecule but can nevertheless be used as a mild oxidant. In some cases, by using a mild oxidizability of carbon dioxide, the activity and selectivity can be markedly improved. However, a small amount of carbon dioxide formed as a by-product in EB dehydrogenation is known to inhibit the catalytic activity of commercial catalyst due to the decomposition of active phase in the presence of carbon dioxide (Appl. Catal. 26, 65 (1986); Appl. Catal., 67, 179 (1991)).
However, in the recent years, it has been disclosed that carbon dioxide may act in a positive manner in the dehydrogenation of ethylbenzene. Sugino et al. reported that the activity of dehydrogenation of ethylbenzene was significantly improved under the flow of carbon dioxide by means of a catalyst having an active carbon carrier impregnated with lithium ferrie. Sugino et al. reported that the source of the enhancement of activity was attributable to the oxidative dehydrogenation activity of ethylbenzene via carbon dioxide as an oxidant (Appl. Catal., 121, 125 (1995)). Nozaki et al. observed that the enhancement effect of the dehydrogenation activity of ethylbenzene based on carbon dioxide under a Na2O/Al.sub.2 O.sub.3 basic catalyst. In latter case, it was explained that the enhancement of a catalytic activity was due to a shift in the reaction equilibrium towards styrene, attributable to the simultaneous occurrence of the dehydrogenation reaction of ethylbenzene and the reverse water-gas shift reaction which had the effect of converting hydrogen so produced by carbon dioxide (Appl. Catal., 37, 207 (1988))
As explained above, various methods have been proposed in order to improve the dehydrogenation reaction of ethylbenzene with steam. In the case of the dehydrogenation reaction of ethylbenzene using carbon oxide, however, carbon dioxide works to deactivate the commercial catalyst therein. Consequently, in order to efficiently use carbon dioxide as a mild oxidant, it becomes necessary to design a new kind of catalyst which can promote the dehydrogenation reaction of ethylbenzene without decomposition of a catalyst by carbon dioxide.